Analog On Demand Feature

The Analog On Demand feature allows the ADC and the AC analog peripherals to automatically enable the OPAMPx only when it is needed, thereby allowing a reduction in power consumption. It also allows the ADC analog block to be powered-off when a conversion is completed.
Note: The Analog On Demand is independent from the On Demand Clock request feature, which is used by peripherals to automatically request a source clock which was previously stopped.
OPAMP case

The Analog On Demand feature of the OPAMPx is activated by writing a '1' to the OPAMP.OPAMPCTRLx.ONDEMAND bit.

In that case, the OPAMPx is automatically enabled when the ADC or the AC requests it (as an input) and is automatically disabled when no more requests are coming from these peripherals.
CAUTION: The Analog On Demand feature is not fully supported on cascaded OPAMPs. If several OPAMPs are cascaded together, only the OPAMPx that is connected to the ADC or AC can be enabled/disabled automatically. Upstream OPAMPs will not benefit from this feature.

In Standby Sleep mode, the Analog On Demand feature is still supported if OPAMP.OPAMPCTRLx.RUNSTDBY=1.

If OPAMP.OPAMPCTRLx.RUNSTDBY=0, the OPAMPx will be disabled entering this Sleep mode.

ADC case

For the ADC peripheral, Analog On Demand feature is enabled by writing the ADC.CTRLA.ONDEMAND bit to '1'.

When this feature is activated, the analog block is powered-off when the conversion is complete.

In Sleep mode, when an ADC start request is detected, the analog block is powered-on again and the ADC starts a new conversion after the start-up time delay.
Note: If the OPAMPx is set to accept Analog On Demand requests but the ADC is not, the ADC will send continuous requests to the OPAMPx keeping it enabled until the ADC is switching on another input.
AC case

For the AC peripheral,there is no explicit ONDEMAND bit.

Analog On Demand requests are issued either when the AC is used in Single-Shot mode, or when comparisons are triggered by events from the Event System.